FIG. 1a shows a cross section through a typical pixelated OLED display device 10. This comprises a substrate 12 bearing a transparent conductive oxide layer 14, typically ITO (Indium Tin Oxide), which may be patterned, typically around 40 nm in thickness. Over this is deposited a hole injection layer (HIL) 16 typically comprising a conducting polymer such as PSS:PEDOT (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This helps match the hole energy levels of the ITO anode and light emitting polymer (and can also assist in planarising the ITO), and is typically around 30 nm in thickness though potentially up to around 150 nm. A similar layer is generally present in an organic photovoltaic device to facilitate the extraction of holes. Commercial hole injection materials are available, inter alia, from Plextronics Inc.
The hole injection layer is, in this example, followed by an intermediate polymer layer, interlayer (IL) 18—also known as a hole transport layer (HTL). This is made of a hole transport material which allows efficient transport of holes; it typically has a thickness in the range 20 nm to 60 nm and is deposited over the hole injection layer and, generally, is cross-linked so that it is insoluble in the solvent used to deposit the subsequent layer. One example material from which the interlayer may be fabricated is a co-polymer of polyfluorene-triarylamine or similar (examples of other suitable materials are described by Bradley et al. in Adv. Mater. vol 11, p241-246 (1999) and in Chapter 2 of Li and Meng—see below).
Over this is deposited one or more layers of light emitting polymer (LEP) 20 to form an LEP layer or stack; a typical example of a light emitting polymer is PPV (Poly(p-phenylenevinylene)). A cathode 22 is deposited over the LEP stack, for example comprising a layer of sodium fluoride (NaF) followed by a layer of aluminium. Optionally an additional electron transport layer may be deposited between the LEP stack 20 and cathode 22.
It can be advantageous to arrange for the electrodes and the layers between to define a resonant cavity as described, for example, in WO00/76010. Such an arrangement can improve the efficiency of operation of the device and increase the colour gamut available.
The device illustrated in FIG. 1a is a bottom-emitting device, that is light generated in the LEP stack is coupled out of the device through the substrate, via the transparent ITO anode layer (in an active matrix display a thin film transistor (TF) may be located in one corner of the pixel). It is also possible to fabricate top-emitting devices using a thin cathode layer (in which case a thick metal anode may be employed), for example a layer of ITO or zinc oxide less than around 100 nm in thickness. Although the structure of FIG. 1a shows an LEP stack the same basic structure may also be employed for small molecule (and dendrimer) devices.
The materials to fabricate an OLED or other organic electronic device may be deposited by ink jet printing (IJP). As illustrated, for this type of pixelated device the materials may be deposited in solution from an ink jet print head into regions defined by banks 24 (or, equivalently, wells).
The skilled person will appreciate that there are many variants of an organic electronic device fabrication process in the context of which the techniques we have described may be employed. For example, the ITO layer may be omitted and instead the hole injection layer 16 used as the anode layer. Additionally or alternatively the electrical conductivity of the hole injection layer 16 may be supported by an underlying metallic grid (which may optionally be transparent by using fine grid lines and/or thin metal). Such an approach may be employed, for example, in an OLED lighting tile with a large area of coverage and connections at the edge. In the case of a lighting tile, generally large-area deposition techniques such as evaporation are employed in preference to ink jet deposition and the pixel-defining banks/wells are absent. Optionally a flexible substrate such as PET (polyethylene terephthalate) or polycarbonate may be employed.
FIG. 1b shows a view from above of a portion of an example three-colour active matrix pixelated OLED display 200 after deposition of one of the active colour layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display. In a colour display different coloured (sub)pixels may comprise respectively green, red and blue light emitting polymer layers.
Organic electronic devices provide many potential advantages including inexpensive, low temperature, large scale fabrication on a variety of substrates including glass and plastic. Organic light emitting diode displays provide additional advantages as compared with other display technologies—in particular they are bright, colourful, fast-switching and provide a wide viewing angle. OLED devices (which here includes organometallic devices and devices including one or more phosphors) may be fabricated using either polymers or small molecules in a range of colours and in multicoloured displays depending upon the materials used. For general background information reference may be made, for example, to WO90/13148, WO95/06400, WO99/48160 and U.S. Pat. No. 4,539,570, as well as to “Organic Light Emitting Materials and Devices” edited by Zhigang Li and Hong Meng, CRC Press (2007), ISBN 10: 1-57444-574X, which describes a number of materials and devices, both small molecule and polymer. (Here “small molecule” refers to non-polymeric small molecules—some so-called small molecules such as dendrimers end may be relatively large, but nonetheless have the characterizing feature that they do not comprise multiple repeat units assembled by polymerization).
Despite the aforementioned advantages of OLEDs there are some problems in efficiently manufacturing OLEDs in a production process. This is because, in general, there is a desire to achieve repeatable performance to relatively tight tolerance limits, in part driven by the human eye's colour and luminance sensitivity. The performance of an OLED can be modelled theoretically but in practice there are many parameters in such models which are not well known and underlying assumptions which mean that this approach is not reliable. On the other hand, repeatedly adjusting the parameters of a production process until devices with the required characteristics are produced is slow and expensive (since many substrates are discarded). There is therefore a need for improved techniques.